In the known art, there are ample descriptions of catalytic systems with three components, based on derivatives of lanthanides (rare-earth), for the polymerization of butadiene having a polymer with a high 1,4-cis chain units.
For example German patents 1812935, 2011543, 2833721, 2830080, Chinese patent 85101199 and International PCT patent 93-05083, describe the preparation of polybutadiene with a high degree of 1,4-cis with the use of catalytic systems based on compounds of rare-earth and aluminium trialkyls.
In all cases the presence of a halogenating agent is essential, generally a derivative of boron having the general formula BRn.sub.n X.sub.3-n, of aluminium having the formula AlR.sub.n X.sub.3-n, silicon having the general formula SiR.sub.n Cl.sub.4-n, wherein R is an alkyl radical and X is a halogen atom such as Cl, Br or I.
The type of organometallic compound of aluminium or halogen Used can influence the activity of the catalyst and the molecular weight of the final polymer but the stereospecificity is not influenced in any way.
The examples provided in the patents of the known art mentioned above indicate that the catalytic systems can be prepared both with the well-known technique "in situ" or preformed, with or without an aging period, before their use in the polymerization reaction.
Interesting examples of preformed catalytic systems with an easy industrial preparation and having a high activity are those described in European patents 201962, 201979 and 207559, wherein an alkyl halide, for example t-Butylchloride, is used as halogenating agent.
All of the documents of the known art quoted so far clearly demonstrate the production of polymers with a high degree of 1,4-cis units, starting from preformed catalytic systems, or systems prepared "in situ", comprising an aluminium trialkyl, a lanthanide salt and a halogenating agent.
The known art also describes the use of binary catalytic systems based on lanthanide salts and aluminium trialkyls. In the latter case the stereospecificity of the final polymer greatly depends on the type of lanthanide salt. In this way, by using lanthanide halides as such or in the form of complexes with alcohols, amines, organic acids or alkylphosphates, together with aluminium trialkyls, polydiolefins are obtained, which, in the case of butadiene, have chains of monomeric units of the 1,4-cis type of over 90% (typically 95-98%).
Valid examples of catalytic systems of this type are disclosed in Japanese patents 8361107 and 84113003, U.S. Pat. No. 4,575,538, German patent 243034, Chinese patent 1036962 and Russian patent 675866 respectively.
When the binary system is obtained starting from an aluminium trialkyl together with an oxygenated lanthanide salt, with the rigorous exclusion of any form of halogenating agent, either organic or inorganic, the final polymer is a polybutadiene with a high content (74-90%) of 1,4-trans units. Examples of these catalytic systems are claimed in patents EP 091287 and JP 9060907, which describe the use of binary systems based on a lanthanide carboxylate and an alkyl derivative of magnesium (MgBut.sub.2) or lithium (Bu--Li) together with an aluminium trialkyl (AlEt.sub.3) respectively. In both cases, apart from the use of costly components such as the alkyls of lithium or magnesium, the catalyst has a limited activity and requires long polymerization times (24 hrs) to reach industrially acceptable conversions. In addition, only plastomers with a high 1,4-trans degree can be obtained without the possibility of changing the stereospecificity of the polymer to higher values of 1,4-cis units typical of an elastomer.
The documents of the known art mentioned above clearly indicate that elastomeric polybutadienes with a high degree of 1,4-cis units can only be obtained with ternary or binary systems when halogen atoms in the form of organic or inorganic halogenating agents are present in the catalytic mixture, or starting from halogenated salts of lanthanides. Without these, only polymers with a high degree of 1,4-trans units (&gt;75%) can be obtained and it is not possible to vary the relative percentage value of 1,4-Trans and 1,4-Cis units.